Lutetium oxide, expressed by Lu2O3 (hereinafter referred to as lutetia) has a cubic crystal structure and no birefringence. This means that it is possible to obtain a sinter with excellent translucency by completely removing segregation caused by pores or impurities. Lutetia has a melting point of over 2490° C., and is known to be a material with excellent heat resistance. Furthermore, because of its high thermal conductivity, it holds promise as a solid state laser host material, and its theoretical transmissivity is approximately 82%. However, lutetia is far more expensive than other rare earth oxides, and consequently almost no research has gone into methods for producing single crystals thereof. Also, because of its extremely high melting point, it is difficult to synthesize large crystals with excellent optical properties with existing single crystal synthesis technology.
Meanwhile, ceramics (polycrystalline substances) can be synthesized at relatively low temperatures below the melting point, so there has for some time now been considerable research into yttrium oxide (yttria) and other rare earth oxides with a high melting point, in an effort to apply these materials to infrared high-temperature window materials, discharge lamp envelopes, corrosion-resistant components, and so forth. In regard to the sintering of polycrystalline, translucent rare earth oxides, the inventors have proposed a method in which aluminum is added as a sintering auxiliary in an amount of 5 to 100 mass ppm to a rare earth oxide (Japanese Laid-Open Patent Application 2003-89578). Nevertheless, the inventors have discovered that controlling the addition of aluminum is not easy with this method because the aluminum is added in such a tiny amount, and in some cases laser oscillation may not be achieved, for example, among other problems.
In addition, let us describe the raw material powder for a rare earth oxide. Oxalates in the form of mother salts most often used as the raw material powder for rare earth oxides. The raw material powders obtained by calcining these oxalates are composed of highly aggregated secondary particles and their particle size distributions are not uniform. Accordingly, packing by molding can not be accomplished sufficiently, and it is not easy to produce high density bodies. To improve this point, methods for manufacturing transparent bodies by low temperature sintering and using easily sinterable raw material powders have been disclosed in recent years (see, for example, Japanese Laid-Open Patent Applications H9-315865 and H11-278933). With these methods, powders whose particle size distributions are relatively uniform and which undergo little aggregation, which are obtained by using carbonates as the mother salts because they can be fired at lower temperatures, and then calcining these, are used as the starting raw material. However, thoroughly eliminating pores during sintering is essential to increasing the transmissivity of a sinter, and the highest linear optical transmissivity that can be attained with just improvements to the raw material powder of a rare earth oxide is about 70%. A sintering auxiliary is necessary to obtain higher transmissivity.